Gingrich's return shows he and his wife, Callista, owed federal taxes of $994,708 on an adjusted gross income of $3,142,066 for 2010, The Washington Post reported.
The couple's income included $450,245 in wages; $41,625 in speaking and board of directors' fees; and $2,525,683 from partnerships and S corporations, the newspaper said.
During the debate Thursday, Romney said he will release his 2011 return but stopped short of saying he would release returns from other years.
"When they're completed this year in April I'll release by returns for last year, and probably for other years," Romney said.
"I don't know how many years I'll release, he said.
Gingrich, who promised to keep the tax issue alive during Thursday's 2-hour debate in Charleston, S.C., said Wednesday he paid almost a third of his earnings to the federal government, unlike the former Massachusetts governor, who said Tuesday he paid about 15 percent in income taxes.
Former U.S. Sen. Rick Santorum of Pennsylvania said he would release his returns too, while Texas Gov. Rick Perry, who has already released his returns, said Republican voters needed to vet Romney's tax records to determine if he would "get eaten alive" against President Barack Obama.
Obama reported paying an effective federal tax rate of 26 percent on his 2010 family income.
One of Romney's best-known supporters, New Jersey Gov. Chris Christie, urged Romney to release his tax returns "sooner rather than later."
Romney, who has said he would release the returns sometime around April, has received "direct private pressure" to deal with the issue immediately, Republicans close to the campaign told The Washington Post.
The conservative National Review called for the returns' swift release, describing the former Massachusetts governor's response to a question about the matter in Monday's Myrtle Beach, S.C., debate, "the weakest moment in his weakest debate."
A Rasmussen Reports daily presidential tracking poll of likely voters, taken Tuesday after the debate, shows a dramatic rebound nationally for Gingrich. It puts Romney at 30 percent, Gingrich at 27 percent, Santorum at 15, Rep Ron Paul of Texas at 13 and Perry at 4 percent.
Perry suspended his campaign Thursday and endorsed Gingrich.
The poll has a margin of error of 3 percentage points.
A CNN/Time/ORC International poll released Wednesday indicates 33 percent of likely Republican South Carolina primary voters back Romney -- a 10 percentage point lead over Gingrich. Romney had a 19 point lead two weeks ago.
Santorum has 16 percent, Paul has 13 percent and Perry has 6 percent, the poll indicates.
The sampling error is 4.5 percentage points.
Romney sought to return fire on Gingrich Wednesday, calling the former speaker's tenure in Washington "leadership by chaos" and likening him to former Vice President Al Gore.
"It's the private sector that creates jobs," Romney said at a Spartanburg, S.C., rally. "Congressmen taking responsibility or taking credit for helping create jobs is like Al Gore taking credit for the Internet."
In Rock Hill, S.C., he referred to Gingrich's attacks on his record at his buyout firm, Bain Capital LLC, saying, "It's not the way to build a strong economy to tear down fellow Americans or to attack capitalism."
Gingrich told a crowd in Warrenville, S.C., to watch out for the Romney campaign between now and Saturday, when the primary is held.
"I think they will be unendingly dirty and dishonest ... because they're desperate," he said. "I think they have internal polls that show them losing."
Leading off the GOP candidate debate in South Carolina Thursday, CNN reporter John King asked Gingrich if he'd care to respond to his former wife's claim.
"No," Gingrich said, before saying he would respond anyway.
Gingrich drew applause from the studio audience when he said he was "appalled" King would start the debate by raising the question of his marital fidelity.
"Every person in here knows personal pain ... to take an ex-wife and make it two days before the primary a major issue is as close to despicable as anything I can imagine," he said.
Gingrich called the allegation by his ex-wife "trash" and false.